I/O circuit with phase mixer for slew rate control

ABSTRACT

An apparatus includes a terminal, a first plurality of driver lines, and a first phase mixer. The driver lines drive the terminal to a first logic state responsive to a first enable signal. The first phase mixer is coupled to a first one of the first plurality of driver lines. The first phase mixer is operable to receive the first enable signal and a first delayed enable signal derived from the first enable signal and generate a first signal on the first driver line having a first configurable delay with respect to the first enable signal by mixing the first enable signal and the first delayed enable signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/041,268, filed Mar. 3, 2008, and issued as U.S. Pat. No. 7,764,554 onJul. 10, 2010. This application and patent are incorporated herein byreference, in their entirety, for any purpose.

TECHNICAL FIELD

Embodiments of the disclosed subject matter relate generally to timingcontrol of integrated circuit devices and, more particularly, to an 110circuit including a phase mixer for slew rate control.

Modern integrated circuit devices are comprised of millions ofsemiconductor devices, e.g., transistors, formed above a semiconductorsubstrate, such as silicon. These devices are very densely packed, i.e.,there is little space between them. Similarly, densely packedelectrically conducting lines may also be formed on the semiconductorsubstrate. By forming selected electrical connections between selectedsemiconductor devices and selected conducting lines, circuits capable ofperforming complex functions may be created. For example, bits of datamay be stored by providing electrical current to a plurality of bitlines and an orthogonal plurality of word lines that may be electricallycoupled to one or more capacitors in a semiconductor memory.

Semiconductor memory devices are one type of integrated circuit devicethat has been widely used to enhance the performance of computersystems, especially when executing memory intensive applications.Semiconductor memory devices typically include an array of memory cells,address decoding circuitry for selecting one of or a group of the memorycells for reading or writing data, sensing circuitry for detecting thedigital state of the selected memory cell(s), and input/output lines toreceive the sensed data and convey that information for eventual outputfrom the semiconductor memory. Synchronous memory devices receive anddeliver data coincident with a clock signal. Typically, an externalclock signal is received by the device. Input signals and data arereceived synchronized with the external clock signal. Due to internaldelays associated with the components of the memory device, the phase ofthe external clock signal is shifted within the memory device. Timingcontrol circuits, such as delay-locked loops (DLLs) or phase lockedloops (PLLs), may be provided for synchronizing the output signals andthe output strobe signals with the provided external clock signal.

Another aspect of timing control relates to controlling the rate atwhich the values of the outputs change during transitions from one logicstate to another. This rate of change, commonly referred to a slew rate,is typically the subject of one or more performance specifications for amemory device. Common output driver circuits employ a plurality ofindividual driver lines that each feed each individual output pad orpin. For example, a driver output circuit may have 4, 8, or 16 driverlines per output line, also referred to as a DQ line. The timingarrangement for firing the individual driver lines affects the net slewrate for the output line. By delaying one line with respect to anotherthe slew rate may be decreased. Typically, such timing control is doneby adding inverter delays to the output path. The final timingconfiguration is typically static. A limitation of this type of timingcontrol is that only a relatively coarse level of timing control isachievable. Because the logic level cannot be changed, a pair ofinverters must be used for each time delay element. Hence, the smallestdelay that can be imposed is typically two inverter delays. This aspectof timing control can also be applied to controlling the slew rateassociated with turning on or off on-die termination (ODT) circuits toimprove signal integrity during the receipt of input data on the DQlines.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The embodiments of the disclosed subject matter will hereafter bedescribed with reference to the accompanying drawings, wherein likereference numerals denote like elements, and:

FIG. 1 is a simplified block diagram of a computer system;

FIG. 2 is a simplified block diagram of data output circuitry in amemory device of the system of FIG. 1; and

FIG. 3 is a simplified circuit diagram of a phase mixer in the dataoutput circuitry of FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

One or more specific embodiments of the disclosed subject matter aredescribed below. It is specifically intended that the disclosed subjectmatter not be limited to the embodiments and illustrations containedherein, but include modified forms of those embodiments includingportions of the embodiments and combinations of elements of differentembodiments as come within the scope of the following claims.

Embodiments of the disclosed subject matter are described with referenceto the attached figures. Various structures, systems and devices areschematically depicted in the drawings for purposes of explanation onlyand so as to not obscure the embodiments of the disclosed subject matterwith details that are well known to those skilled in the art.Nevertheless, the attached drawings are included to describe and explainillustrative examples of embodiments of the disclosed subject matter.The words and phrases used herein should be understood and interpretedto have a meaning consistent with the understanding of those words andphrases by those skilled in the relevant art. No special definition of aterm or phrase, i.e., a definition that is different from the ordinaryand customary meaning as understood by those skilled in the art, isintended to be implied by consistent usage of the term or phrase herein.To the extent that a term or phrase is intended to have a specialmeaning, i.e., a meaning other than that understood by skilled artisans,such a special definition will be expressly set forth in thespecification in a definitional manner that directly and unequivocallyprovides the special definition for the term or phrase. Nothing in thisapplication is considered critical or essential to the disclosed subjectmatter unless explicitly indicated as being “critical” or “essential.”

Referring now to the drawings wherein like reference numbers correspondto similar components throughout the several views and, specifically,referring to FIG. 1, embodiments of the disclosed subject matter shallbe described in the context of a simplified block diagram of a computersystem 100. The computer system 100 includes a microprocessor 110coupled to a memory controller 120. The memory controller 120 is coupledto a memory device 130. The microprocessor 110 issues commands to thememory controller 120 to access data stored in the memory device 130.For clarity and to avoid obscuring the instant invention, only thoseinterconnections and modules related to the control of the timing in thememory device 130 are illustrated. The memory device 130 includes amemory array 140 for storing data and data output circuitry 150 foroutputting data read from the memory array 140 on a data output path160.

The memory controller 120 provides the input clock signal (CLKIN) andinput strobes (DQSIN) to the memory device 130. A timing control circuit170 receives the input clock signal and generates the output strobes(DQSOUT) For typical memory systems, the DQSIN and DQSOUT signals can betransferred in different time through the same bidirectional DQS line.For a high-speed memory interface, the may be separated intounidirectional DQS lines. The timing control circuit 170 may beimplemented using a variety of circuit types, such as closed loop timingcircuits (e.g., delay locked loop, phase locked loop), open loop timingcircuits (e.g., measure control delay), or a combination of both (e.g.,an MCD loop embedded within a DLL loop). Generally, the timing controlcircuit 170 employs a delay model 172 of the data output circuitry 150for establishing the timing control. Those of ordinary skill in the artare familiar with timing control circuitry for synchronizing the CLKINand DQS/DQ signals, including the use of the delay model 172, so thedetails of such a circuit are not described in greater detail herein.

The memory device 130 may also include on-die termination (ODT)circuitry 180 for driving the DQ terminal 160 to a known state toprevent distortion of data caused by signal reflection during normaloperation (such as a read/write operation) of the memory device 130.

The microprocessor 110, memory controller 120, and memory device 130 maytake on numerous forms, depending on the particular implementation.Those of ordinary skill in the art are knowledgeable regarding theparticular construct and operation of these devices.

As will be described in greater detail below in reference to FIG. 2, thememory device 130 includes adjustable driver circuitry 190 that includesphase mixer circuitry to allow fine resolution control of the slew rateexhibited on the output of the adjustable driver circuitry 190. As willbe described below, the adjustable driver circuitry 190 may beimplemented in the data output circuitry 150, the ODT circuitry 180,and/or the delay model 172.

Turning now to FIG. 2, a simplified block diagram of the adjustabledriver circuitry 190 is illustrated. In the following descriptionlettered suffixes are added in some cases to denote similar elements.Reference to a particular element is denoted by the use of theparticular letter suffix, while reference to the collective group ofsimilar elements is denoted by the use of the numeral without a lettersuffix. For purposes of the following illustration, the adjustabledriver circuitry 190 is shown having four driver lines 200A, 200B, 200C,200D on a pull-up path 205 and four driver lines 210A, 210B, 210C, 210Don a pull-down path 215 for an individual DQ line 220. In FIG. 2, onlythe circuitry associated with the single DQ line 220 is shown. In thecase where the adjustable driver circuitry 190 is implemented in thedata output circuitry 150 or the ODT circuitry 180, the circuitry 190would be repeated for each DQ line 220 of the memory device 130.Moreover, the number of driver lines 200, 210 may also vary (e.g., 2, 3,7, 8, etc.) depending on the particular implementation.

In the illustrated embodiment, two of the driver lines 200A, 200B fromthe pull-up path 205 and two of the driver lines 210A, 210B from thepull-down path 215 are routed to phase mixers 225A, 225B, 235A, 235B,respectively. As will be described in greater detail below, the phasemixers 225A, 225B, 235A, 235B allow fine resolution control of thetiming of the associated driver lines 200A, 200B, 210A, 210B. Theconfiguration of the phase mixers 225A, 225B with respect to the amountof delay imposed is configured using control words CW1A, CW1B,respectively. Similarly, the configuration of the phase mixers 235A,235B with respect to the amount of delay imposed is configured usingcontrol words CW2A, CW2B, respectively. By setting various values forthe control words, the relative timing of the driver lines 200, 210 maybe modified, thereby allowing control of the slew rate for the DQ line220.

In the illustrated embodiment, the driver lines 200C, 200D, 210C, 210Dare illustrated as not having associated phase mixers. In thisconfiguration, the timing of the driver lines 200C, 200D, 210C, 210D iscontrolled in the conventional fashion. The driver lines 200C, 200D,210C, 210D are routed to buffers 240C, 240D, 245C, 245D. Timing adjustcircuitry 250, 255 is provided for making coarse timing adjustments tothe driver line signals. The outputs of the timing adjust circuitry 250,255 are provided to a pull-up driver 260 and a pull-down driver 265,respectively. The timing adjust circuitry 250, 255 may be configured tointroduce variable delays (e.g., based on inverter delays) to the driverlines 200, 210 as is done with conventional driver circuits. The fineresolution timing control provided by the phase mixers 225, 235 allowsfine adjustments to be made to the slew rate seen at the DQ line 220.Although only two driver lines 200A, 200B, 210A, 210B for each path 205,215 are illustrated, it is contemplated that in some embodiments thenumber of driver lines 200, 210 with associated phase mixer delaycontrol may be more or less. For example, in an embodiment with 8 driverlines 200, 210, additional phase mixers 225, 235 may be provided. Foranother example with additional ODT driver lines that look similar tothe output driver lines 200, 210, additional phase mixers may be furtherprovided in an embodiment where the ODT driver lines are embedded in theoutput driver lines.

Each phase mixer 225, 235 receives an early signal (i.e., DQout orDQSout), denoted by the “E” input, and a late signal (i.e., DQout orDQSout), denoted by the “L” input. The early signal for the pull-upphase mixers 225A, 225B is generated by a pull-up enable signal (PUEN)indicating that the DQ line 220 should be driven at a logically highstate, and the early signal for the pull-down phase mixers 235A, 235B isgenerated by a pull-down enable signal (PDEN) indicating that the DQline 220 should be driven at a logically low state. In the illustratedembodiment, the late signal for the phase mixer “A” phase mixers 225A,235A is generated by the “B” phase mixers 225B, 235B, and vice versa.

Using the phase mixers 225, 235 to generate the late signals obviatesthe need to generate the late signals externally, thereby reducing realestate requirements for the adjustable driver circuitry 190.Alternatively, additional circuitry may be provided that receives thePUEN and/or PDEN signals and generates a delayed version of thosesignals for the late signal. Because, the early signal is used toultimately generate the late signal, one of the phase mixers in eachgroup is configured to select only the early signal for controlling thetiming of its associated driver line 200, 210. For example, the phasemixer 225A may be configured using the control word CW1A to select theearly signal (i.e., the PUEN signal). Once, the first edge seen on thePUEN signal propagates through the phase mixer 225A, the late signal isgenerated for the phase mixer 225B. The control word CW1B of the phasemixer 225B may then be set such that the output clock generated has aphase that lies somewhere between the phase of the early signal and thelate signal. Hence, the signal on the driver line 200B would be delayedwith respect to the signal on the driver line 200A. The relative orderin which the driver lines activate may be reversed by setting the phasemixer 225B to select the early signal and setting the phase mixer 225Ato generate the phase-mixed late signal.

Due to process variance, each driver line 200, 210 may have a slightlydifferent characteristic impedance. By controlling the order in whichthe driver lines 200, 210 fire using the timing adjust circuitry 250,255 and the phase mixers 225, 235, the dynamic impedance of the DQ line220 may be set, thereby controlling the slew rate of the DQ line 220.

Turning now to FIG. 3, a simplified diagram of a phase mixer 300suitable for use as one of the phase mixers 225, 235 in the adjustabledriver circuitry 190 is shown. An exemplary phase mixer is described inU.S. Pat. No. application Ser. No. 11/983,201, entitled “High Speed WideFrequency-Range Digital Phase Mixer”, filed on Nov. 7, 2007, assigned tothe assignee of the present application, and incorporated herein byreference in its entirety.

In the illustrated embodiment, the phase mixer 300 is an n-bit phasemixer. The number of stages 305 of the phase mixer 300 corresponds tothe number of bits in the control word (e.g., CW1A, CW1B, CW2A, CW2B inFIG. 2). The number of stages 305 may vary depending on the particularimplementation and the granularity of the control required for the phasemixer 300. The number of stages determines the number of steps that maybe provided in the output signal. If the control word is set to “00 . .. 000”, the early signal is selected by each stage 305. Contrastingly,if the control word is set to “11 . . . 111”, the late signal isselected. Values in between provide incremental delay values between theearly signal and the late signal. Hence, the amount of delay imposed isa function of the number of stages 305 for which the control bit is setto “1”. Of course complimentary logic may be employed such that “1”corresponds to a late signal selection and “0” corresponds to an earlysignal selection.

The phase mixer 300 includes input buffers 310A, 310B for receiving theearly signal and input buffers 315A, 315B for receiving the late signal.The outputs of each stage 305 are provided to output buffers 320, 325.The output buffer 320 provides the delayed signal for the driver line200, 210, and the output buffer 325 provides the late signal for theassociated phase mixer, as illustrated in FIG. 2. An additionalinverting buffer 330 is coupled to the output buffer 325 for the latesignal to provide an even number of inversions so that the late signalis not inverted with respect to the early signal. Again, the arrangementwhere the late signal is generated using the output of the phase mixer300 may not be required in some embodiments. The late signal may bereceived from a different source.

Each stage 305 includes an enable buffer 335 that selects one of buffer340 or buffer 345 for providing the output of the stage 305. The buffer340 is coupled to the buffers 310A, 310B for receiving the early signal,and the buffer 345 is coupled to the buffers 315A, 315B for receivingthe late signal. The enable buffer 335 is coupled to receive a bit ofthe control word, c<0 . . . n>. The enable buffer 335 enables the buffer340 responsive to the associated control bit being “0”, therebyselecting the early signal, and enables the buffer 345 responsive to thecontrol bit being “1”, thereby selecting the late signal.

The input signal received by the output buffer 320 is thus the sum ofthe early or late signal selections configured for each stage 305. Bysetting the control bits, the net slew rate of the DQ line 220 in FIG. 2may be controlled. The configuration of the control bits for setting thetiming relationships of the driver lines 200, 210 may be static ordynamic. In a static arrangement, the control words may be set byfusible elements during a configuration process that compensates forprocess variation within the memory device 130. In a dynamicarrangement, the slew rate of the DQ line 220 may be controlled using aPLL or DLL circuit that automatically adjusts the values of the controlwords to affect change in the slew rate.

In the case where the memory device 130 includes a timing controlcircuit 170 employing a delay model 172 for synchronizing the DQ and DQSwith a reference input clock signal, the delay model 172 is configuredto be consistent with the characteristics of the data output circuitry150 employing phase mixers 300. As described above in reference to FIG.1, the data output path 160 is typically not included in the controlloop used for clock synchronization, but rather the delay associatedwith the output path 160 is modeled. In one embodiment where theconfiguration of the data output circuitry 150 is static with respect tothe determination of the control words, the delay model 172 may be alsoconfigured statically. However, in another embodiment that employsdynamic control of the data output circuitry 150 slew ratecharacteristics, the delay model 172 may also be provided with phasemixers 300 for dynamically modeling the delay. In the delay model 172,the timing of each driver line 200, 210 may be modeled individually. Forthose driver lines 200C, 200D, 210C, 210D without phase mixer control, astatic model may be used. For the driver lines 200A, 200B, 210A, 210Bwith phase mixers 225, 235, duplicate phase mixers may also be providedthat are configured in accordance with the same control words as thephase mixers 225, 235 in the data output circuitry 150. Hence, if thetiming is changed in one of the data output circuitry 150 elements, acorresponding change may also be made in the delay model 172 using theduplicate circuitry.

In the embodiment, where the adjustable driver circuitry 190 is used inthe ODT circuitry 180, the adjustable driver circuitry 190 operatessimilarly to how it operates with the data output circuitry 150, withthe exception of the enable signals. In the case where the adjustabledriver circuitry 190 is used in the data output circuitry 150, the logiclevels of the PUEN signal and the PDEN signals correspond to the desiredlogic level of the output at the DQ terminal 220. For example, when alogic “1” is the desired output, the PUEN and PDEN signals are both at alogic “1”, and conversely, when a logic “0” is the desired output, thePUEN and PDEN signals are both at a logic “0”.

The logic levels for the control of the adjustable driver circuitry 190in the ODT circuitry 180 depends on the ODT scheme used. There are threetypical ODT schemes, VDDQ terminated ODT, ground terminated ODT, andcenter-tapped ODT depending on the desired state of the terminal 220when the ODT circuitry 180 is enabled. In VDDQ terminated ODT the DQterminal 220 is taken to the same logic level as VDDQ. In groundterminated ODT the DQ terminal 220 is grounded. In center-tapped ODT,the DQ terminal 220 is taken to a level approximately equal to half ofVDDQ. The different combinations of PUEN and PDEN determine the ODTstate. If In VDDQ terminated ODT, PUEN is “1” and PDEN is “1”. In groundterminated ODT, PUEN is “0” and PDEN is “0”. In center-tapped ODT, PUENis “1” and PDEN is “0”.

The particular embodiments disclosed above are illustrative only, as thedisclosed subject matter may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of thedisclosed subject matter. Accordingly, the protection sought herein isas set forth in the claims below.

1. An apparatus, comprising: a terminal; and a pull-up path coupled tothe terminal, the pull-up path including: a plurality of pull-up driverlines for driving the terminal to a first logic state responsive to apull-up enable signal; a plurality of pull-up phase mixers coupled to atleast a first subset of the pull-up driver lines, each pull-up phasemixer being operable to receive the pull-up enable signal and a delayedpull-up enable signal derived from the pull-up enable signal andgenerate a signal on the associated pull-up driver line having a firstconfigurable delay with respect to the pull-up enable signal by mixingthe pull-up enable signal and the delayed pull-up enable signal; and apull-down path coupled to the terminal, the pull-down path including: aplurality of pull-down driver lines for driving the terminal to a firstlogic state responsive to a pull-down enable signal; and a plurality ofpull-down phase mixers coupled to at least a second subset of thepull-down driver lines, each pull-down phase mixer being operable toreceive the pull-down enable signal and a delayed pull-down enablesignal derived from the pull-down enable signal and generate a signal onthe associated pull-down driver line having a second configurable delaywith respect to the pull-down enable signal by mixing the pull-downenable signal and the delayed pull-down enable signal.
 2. The apparatusof claim 1, wherein each of the pull-up phase mixers is configurablebased on a control word and includes a plurality of stages, each stagebeing responsive to a bit of the associated control word to select oneof the pull-up enable signal or the delayed pull-up signal as an outputof the stage, and the each pull-up phase mixer is configured to mix theoutputs of the stages to generate the signal on the associated pull-updriver line.
 3. The apparatus of claim 2, wherein each of the pull-downphase mixers is configurable based on a control word and includes aplurality of stages, each stage being responsive to a bit of theassociated control word to select one of the pull-down enable signal orthe delayed pull-down signal as an output of the stage, and the eachpull-down phase mixer is configured to mix the outputs of the stages togenerate the signal on the associated pull-down driver line.
 4. Theapparatus of claim 1, wherein an output of a first pull-up phase mixerin the plurality of pull-up phase mixers provides the delayed pull-upenable signal for a second pull-up phase mixer in the plurality ofpull-up phase mixers and an output of the second pull-up phase mixerprovides the delayed pull-up enable signal for the first pull-up phasemixer.
 5. The apparatus of claim 1, further comprising: at least a firstbuffer coupled to a pull-up driver line not included in the firstsubset; and first timing adjust circuitry coupled to the plurality ofpull-up driver lines and operable to provide a first additionalconfigurable delay on each of the pull-up driver lines, the firstadditional configurable delay having a resolution less than the firstconfigurable delay.
 6. The apparatus of claim 5, further comprising: atleast a second buffer coupled to a pull-down driver line not included inthe second subset; and second timing adjust circuitry coupled to theplurality of pull-down driver lines and operable to provide a secondadditional configurable delay on each of the pull-down driver lines, thesecond additional configurable delay having a resolution less than thesecond configurable delay.
 7. An apparatus, comprising: a plurality ofpull-up phase mixers coupled to a first subset of pull-up driver lines,at least one pull-up phase mixer being operable to receive a pull-upenable signal and a delayed pull-up enable signal based, at least inpart, on the pull-up enable signal and generate a corresponding pull-upoutput signal on an associated pull-up driver line; a plurality ofpull-down phase mixers coupled to a second subset of pull-down driverlines, at least one pull-down phase mixer being operable to receive apull-down enable signal and a delayed pull-down enable signal based, atleast in part, on the pull-down enable signal and generate acorresponding pull-down output signal on an associated pull-down driverline; a pull-up driver configured to drive a terminal to a first logicstate responsive to receipt of the corresponding pull-up output signalon an associated pull-up driver line; and a pull-down driver configuredto drive the terminal to a second logic state responsive to receipt ofthe corresponding pull-down signal on an associated pull-up driver line.8. The apparatus of claim 7, wherein the at least one pull-up phasemixer is configurable based on a control word and includes a pluralityof stages, each stage being responsive to a bit of the associatedcontrol word to select one of the pull-up enable signal or the delayedpull-up signal as an output of the stage, and the at least one pull-upphase mixer is configured to mix the outputs of the stages to generatethe signal on the associated pull-up driver line.
 9. The apparatus ofclaim 7, wherein the at least one pull-down phase mixer is configurablebased on a control word and includes a plurality of stages, each stagebeing responsive to a bit of the associated control word to select oneof the pull-down enable signal or the delayed pull-down signal as anoutput of the stage, and the at least one pull-down phase mixer isconfigured to mix the outputs of the stages to generate the signal onthe associated pull-down driver line.
 10. The apparatus of claim 7,wherein the corresponding pull-up output signal on the associatedpull-up driver line is based, at least in part, on mixing the pull-upenable signal and the delayed pull-up enable signal.
 11. The apparatusof claim 7, wherein the corresponding pull-down output signal on theassociated pull-down driver line is based, at least in part, on mixingthe pull-down enable signal and the delayed pull-down enable signal. 12.The apparatus of claim 7, wherein an output of a second pull-up phasemixer provides the delayed pull-up enable signal for the at least onepull-up phase mixer.
 13. A method for mixing signals, comprising:generating a pull-up enable signal and a pull-down enable signal;generating at least one delayed pull-up enable signal and at least onedelayed pull-down enable signal; mixing the pull-up enable signal andthe at least one delayed pull-up enable signal to generate a pull-updrive signal; mixing the pull-down enable signal and the at least onedelayed pull-down enable signal to generate a pull-down drive signal;driving a terminal to a first logic state based, at least in part, on atleast one of the pull-up drive signal and the pull-down drive signal.14. The method of claim 13, further comprising: generating a bufferedpull-up enable signal based, at least in part, on the pull-up enablesignal.
 15. The method of claim 13, further comprising: generating abuffered pull-down enable signal based, at least in part, on thepull-down enable signal.
 16. The method of claim 13, wherein the delayedpull-up enable signal is generated by a pull-up phase mixer.
 17. Themethod of claim 13, wherein the delayed pull-down enable signal isgenerated by a pull-down phase mixer.
 18. The method of claim 13,further comprising: before driving a terminal, adding a configurabledelay to at least one of the pull-up drive signal and the pull-downdrive signal.
 19. The method of claim 13, wherein the pull-up enablesignal and pull-down enable signal have the same state.
 20. The methodof claim 13, further comprising: generating at least one control word toconfigure at least one of the pull-up drive signal and the pull-downdrive signal.